Porous fluoropolymer materials, and in particular, expanded polytetrafluoroethylene (ePTFE) materials, typically exhibit relatively low elongation when stressed in the direction parallel to the orientation of the fibrils. High strength ePTFE materials have relatively low elongation values compared to lower strength ePTFE materials. Uniaxially expanded materials can exhibit high elongation when stressed in the direction orthogonal to the fibrils; however, the membranes are exceptionally weak in this direction.
Uniaxially expanded ePTFE tubes positioned on mandrels have been mechanically compressed and heat treated to achieve higher elongations prior to rupture. Such tubes also exhibit recovery if elongated prior to rupture and released from stress. U.S. Pat. No. 4,877,661 to House, et al. discloses porous PTFE having the property of rapid recovery and a method for producing these materials. Additionally, the pores of compressed tubes have been penetrated with elastomeric materials. For example, U.S. Pat. No. 7,789,908 to Sowinski, et al. discloses an elastomeric recoverable PTFE material that includes longitudinally compressed fibrils of an ePTFE material penetrated by an elastomeric material within the pores which define an elastomeric matrix.
A need continues to exist for thin, strong membranes that exhibit high degrees of elongation, such as greater than 50% elongation. Some applications further demand qualities such as thinness, low density, and/or small pore size, as well as combinations thereof. Other applications require a relatively low force to elongate the membrane.